1. Field of the Invention
The present invention relates to a bayonet coupling, between two relatively rotatable ring-shaped members, for axially mounting one of the two relatively rotatable ring-shaped members on the other. The two ring-shaped members can be elements of a lens barrel.
2. Description of the Related Art
In a lens barrel such as a photographing lens barrel for camera, it is sometimes required that two ring-shaped members of the lens barrel, one of which is axially mounted on the other, be engaged with each other so that at least one of the two ring-shaped members is rotatable relative to the other while preventing the two ring-shaped members from moving in the axial direction thereof relative to each other. As an example of this structure, a conventional mechanism in which an outer ring-shaped member is axially mounted on an inner ring-shaped member in a bayonet fashion is known in the art. Specifically, an annular groove having a plurality of insertion openings (cutout portions) are formed on an outer peripheral surface of the inner ring-shaped member (or on an inner peripheral surface of the outer ring-shaped member), while a corresponding plurality of bayonet lugs which are engaged in the annular groove through the plurality of insertion openings are formed on an inner peripheral surface of the outer ring-shaped member (or on an outer peripheral surface of the inner ring-shaped member).
In such a conventional mechanism, the number of the bayonet lugs and the number of the insertion openings correspond to each other in a one-to-one relationship. Based on this relationship between the bayonet lugs and the insertion openings, various proposals for the number of the insertion openings (the number of the bayonet lugs), the circumferential widths of the insertion openings (the circumferential widths of the bayonet lugs), the radial height of the insertion openings (the radial height of the bayonet lugs), have been made to prevent the two relatively rotatable ring-shaped members from being disengaged from each other within a maximum range of relative rotation between the two relatively rotatable ring-shaped members. Any of such conventional proposals are based on the established idea of making the number of the insertion openings and the number of the bayonet lugs correspond to each other in a one-to-one relationship. Such an established idea limits the maximum range of rotation between the two relatively rotatable ring-shaped members, and further limits improvements in operability of the two relatively rotatable ring-shaped members. The operability of the two relatively rotatable ring-shaped members deteriorates, e.g., when one of the plurality of bayonet lugs accidentally hits one of the circumferentially opposite ends of one of the plurality of insertion openings within a predetermined range of rotation between the two relatively rotatable ring-shaped members.